1. Field of the Invention
This invention generally relates to systems that measure the performance of high-speed data communications channels. More particularly, the preferred embodiment of the present invention relates to a method and an apparatus for using dual bit detectors to measure bit errors and event occurrences in a data stream.
2. Description of the Related Art
In recent years, the performance of high-speed communication facilities or devices has risen to a point that the ability to accurately measure its quality has become an ever-increasing challenge. In the area of data communication devices, for example, efforts to reduce size and power while increasing the throughput of such devices has increased the possibility of error. Network suppliers, integrators and users alike want assurances that these devices will perform reliably and can accommodate data transmission rates that routinely exceed several gigabits/second. Providing such assurance requires determining the effective error rate of either the data received by or data transmitted through a high-speed communications channel or device.
One approach of evaluating the performance of a high-speed communications channel device or system is to use a bit error rate tester (BERT). Here, a predetermined bit pattern is transmitted through the target channel, device or system and on to the BERT. The difference between the known transmitted bit pattern and the expected bit stream is considered an error and is accumulated by the BERT to determine the effective bit error rate (i.e., the fraction of the received or transmitted bits that are in error) of the target device or system. Even though bit error rate testing (BER) is a relatively significant measure of performance, merely measuring the error rate of a high speed communications device or system does not provide enough data to characterize the behavior of a communications channel, device or system. As a result, instruments such as BERTs and oscilloscopes have the functionality of creating an eye diagram, jitter measurements, Q-factor measurements, voltage histograms, time histograms and mask tests to facilitate characterizing a high-speed communications device or system during the various stages of development. Here, various techniques that depend on knowing either the information on mismatches between the expected data stream and the received data stream or information about the occurrence of multi-valued voltages representing the data stream landing inside various time or voltage windows within the data eye diagram are the building blocks to better and more-detailed analysis methods. For production or manufacturing environments, generating a comprehensive series of tests that incorporate generating these various analysis techniques for each new component or communications subsystem is too costly and quite time consuming.
There are several commercially available testing systems that characterize and validate the performance of a data signal from a device or communications subsystem using bit error rate measurement methods. These include instruments from Agilent Technologies, Anritsu, SHF; eye diagram-creating oscilloscopes from Agilent Technologies, Tektronix, Lecroy Corporation; and jitter analyzers from Agilent Technologies, Wavecrest, and LeCroy Corporation. As understood, in each of these systems, non-overlapping techniques are used to create various methods for analyzing the quality of a digital communication stream, but, in no case, is a single instrument capable of performing measurements needed to do all the different desired measurements, nor are measurements always allowed in non-constrained (i.e. live) data streams.
What is needed is a single device that includes dual detectors having a dual bit decision tree enabling the user to perform comparative testing without the use of constrained references. What is also needed is a single such device that includes such dual detectors having a dual bit decision tree that can analyze live data.